The desire for on-demand, variable thrust engines is increasing because several evolving tactical missile systems require multi-role, multi-mission, and multi-platform capability and high maneuverability during the end game to kill evading targets. Conventional solid propulsion systems cannot provide on-demand, variable thrust. Pintle-controlled solid and throttling liquid and gel engines provide on-demand variable thrust, but at the cost of propellant usage efficiency. Controlling thrust by turning on and off liquid and gel engines (pulse modulation) also provides variable thrust at a cost to propellant usage efficiency, but at a somewhat lower degree than throttling engines. Multiple liquid or gel engines, which can be turned on and off in various combinations, provide incrementally variable thrust, but at higher cost.
Conventional solid propulsion systems can only change thrust by a change in grain formulation and/or design or by multiple grains that can be fired sequentially. This, however, limits thrust flexibility, increases complexity, cost, and the motor can only produce the pre-defined thrust schedule once a grain has been ignited. One method to minimize the thrust limitation is to use a pintle to increase or decrease the throat area. A variation of this is a variable flow valve that increases or decreases the area through which the motor effluents can pass. Both of these methods can produce continuously variable thrust levels, however, they have a lower propellant usage efficiency when they are operating at reduced thrust. To decrease thrust, these methods open the throat area, which lowers the pressure, which, in turn, decreases the burning rate and, hence, the mass flow rate. The specific impulse (Isp, thrust divided by mass flow rate) decreases as the pressure is reduced.
Liquid and gel engines are also designed with fixed nozzle throats that operate at an optimum pressure. A throttling engine operates by controlling the flow of propellants through a throttling valve. When the flow rate of propellants decreases, the combustion pressure decreases, which decreases both thrust and Isp. When a liquid or gel engine varies thrust by pulse modulation, it operates the engine at the designed pressure the percent of time required to obtain the desired thrust level. There is a finite rise and fall time for each pulse which results in small regions/areas of lower Isp. The overall Isp loss increases as the system requirements demand shorter, more frequent pulses.
A multiple engine liquid or gel propulsion design greatly reduces the loss of Isp during reduced thrust operation by operating only those engines required to reach the design thrust. This design is particularly attractive when the boost and nominal sustain thrusts are significantly different. Some losses in Isp occur during engine start-up and shut-down, but since the engines are not pulsing, these losses are minimal. Problems associated with a multiple engine design are increased manufacturing cost and minimizing system volume.
The above described approaches for achieving on-demand, variable thrust performances are recognized as having at least one common end relationship where providing variable thrust results in a cost to propellant usage efficiency. With the use of multiple liquid or gel engines, which can be turned on and off in various combinations to provide incrementally variable thrust, additional costs in addition to lower propellant usage efficiency are encountered as a result of multiple combinations which are higher in total cost. Solid propellant systems result in lower propellant usage efficiency when the systems are operated at reduced thrust. Changing thrust by a change in grain formulation and/or design or by using multiple grains that can be fired sequentially means that the solid rocket motor can only produce a pre-defined thrust schedule once a grain has been ignited. One approach to overcoming this limitation is to employ a pintle to increase or decrease the throat area. A variation of the pintle system is to employ a variable flow valve that increases or decreases the area through which the motor effluents can pass. Although these described combinations and methods of operation can produce continuously variable thrust levels, they result in low propellant usage efficiency when they are operating at reduced thrust.
Advantageous would be a system which combines a pressure controlled pintle with a throttling liquid, gel, or hybrid engine.
A primary object of this invention is to provide a pressure controlled pintle in combination with a throttling liquid, gel, or hybrid engine.
Another object of this invention is to provide a working relationship between a pressure controlled pintle and a throttling liquid, gel, or hybrid engine to retain a design pressure over a wide thrust range thereby maintaining the engine efficiency at an optimum value.
A further object of this invention is to eliminate the drop of specific impulse (Isp) due to pressure drops generally encountered in prior art combinations since the design pressure is retained by the cooperative relationships between the pressure controlled pintle and the throttling fuel system.
Still a further object of this invention is to keep the cost of the system of this invention reasonable due to requirement of minimizing the number of engines per system.